1. Field of the Invention
This disclosure relates to analog-to-digital converters (ADC), and more particularly, to successive approximation circuits and methods for performing ADC functions on non-sampled or moving signals.
2. Description of the Related Art
Successive approximation is a method of analog-to-digital conversion in which the signal to be converted (i.e. an unknown quantity) is compared repeatedly against a group of weighted references which are chosen from a digital estimation circuit under program control. Typically, the references are weighted in binary sequence, i.e. each weight is twice that of the next smallest, so that the outcome of the completed conversion is directly obtained from the state of the estimation array at the end of a conversion.
The algorithm, or program, used to select from among the weighted references is similar to that used to weigh an object on a balance using a set of reference weights. These weights are tried in order starting with the heaviest, which by definition is a half scale initial guess. Any weight which tips the scale is removed, and at the end of the process, the sum of the weights remaining on the scale is within one smallest reference unit of the unknown's actual weight.
In prior successive approximation A/D converters it is assumed that the unknown value does not change during the course of the conversion. Accordingly, a sample and hold circuit is used to "freeze" the value of the unknown when conversion is in process.
If the unknown is allowed to continue changing during conversion, the normal successive approximation process must be modified to enable the converter to follow the unknown signal and to compensate for possible changes in the unknown between conversion steps. Furthermore, the digital estimate must be constructed in such a way that a minimum number of circuit elements switch at the end of conversion when the smallest values are being determined. This is because circuit switching is unavoidably accompanied by electrical noise, and such noise tends to interfere with the circuits which generate the reference and which compare the reference to the unknown, thus limiting the resolution of the converter.